CHAPTER 8 



Respiration and Metabolism 



AVAILABILITY OF OXYGEN 



T 



■ HE DIRECT DEPENDENCE of animals On an adequate oxygen supply has 



■ been recognized only since the seventeenth century, when Mayow 

 W and Hooke demonstrated the need of animals for air, and later 



when Priestley and Lavoisier discovered the chemical nature of oxygen and 

 the importance of oxidation as the source of energy within the body. 



Wide-spread distribution of oxygen permits life in a great variety of environ- 

 ments under aerobic conditions, exceptions being in deep lakes and in the 

 intestinal tracts of vertebrates, where anaerobic respiratory mechanisms are 

 not uncommon. Oxygen not only is widely distributed but remains fairly 

 constant in amount within a given environment. In some lakes, however, the 

 oxygen supply may be suddenly depleted by plant life and bacteria. Indeed, 

 sudden reductions in numbers of animals have been cited in European waters 

 where, as near the Messina Straits, the paucity of oxygen in the great depths 

 allows hydrogen sullide from decomposing animal bodies to accumulate to 

 such a degree that, on agitation of the waters during storms, the surface fauna 

 is exposed to toxic concentrations of hydrogen sulfide with disastrous results. 



Various animals have evolved diflferent mechanisms for the procurement of 

 oxygen, and, where the supply is particularly limited, emergency adaptations 

 have developed. The responses of organisms to oxygen stress constitute one of 

 the most interesting contributions to adaptive physiology. In swamps of the 

 Paraguayan Chaco, for example, where little or no oxygen can be detected in 

 the subsurface water, a number of adaptations are found in fish for the pro- 

 curement of oxygen from either the air or the surface water-layer. ^- Five 

 different types of aerial respiratory systems have been developed to provide the 

 bulk of oxygen for these fish: pharyngeal diverticular "lungs" {Lepidosiren and 

 Symhranchiis) , gas bladder (Erythrimis), gill lamellae {Hypoponnis), intestinal 

 modifications (Callichthys and Hoplosternxini), and gastric mucosa (Ancistrns). 



The adequacy of the oxygen supply is a function of the number of molecules 

 of the gas a\'ailable to the organisms, whether as molecular oxygen dissolved in 

 water or as molecules of oxygen in air. The amount of oxygen available to the 

 organism— atmospheric or in solution— depends on the partial pressure exerted 

 by the gas, the value of the partial pressure or oxygen tension being directly 

 proportional to the volume per cent of the gas present. Partial pressures of 

 the important gases of dry air at sea level are given in Table 32. 



The amount of gas in solution depends on the absorption coefficient and is 

 proportional to the partial pressure exerted by the gas. The quantity of gas 

 in solution decreases with elevation of temperature and with the addition of 

 solutes to the solvent. Solubility data for oxygen in water, in salt solution 



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